Fixes Dead Field Next to "+" Label in Steps View

What it does
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Allows the user to enter a new step directly by entering text into the text field next to the "+" step and hitting enter.

Notes:
- There is a slight hack going on to check if the step is the step with the "+" label, it checks the index of the NSNotification and checks if it is one greater than the number of registered steps. I did it this way because the new step doesn't "exist" yet in the model and therefore it crashes when I try to access it's label. If either of you guys have a suggestion please let me know.
- It takes a couple seconds to update after pressing enter, I assume this is a network issue.

How to test
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- Create some new steps via this method, make sure they persist.

- [x] Test
- [x] Review

@joshpc @jules2689